Sequence Diagram Message Lines Rendered with stroke="none"

[FrankLedo]

Environment

• mermaid-cli version: 11.4.0, 11.10.1, 11.12.0 (all tested)
• Node version: 22.18.0
• OS: macOS 25.0.0 (Darwin)
• Output format: SVG

Description

When generating sequence diagrams with mmdc (mermaid-cli), the message lines (arrows between participants) are rendered with stroke="none" as an inline attribute, making them invisible in the output.

Steps to Reproduce

1. Create a simple sequence diagram:

sequenceDiagram
    participant A as Alice
    participant B as Bob
    A->>B: Hello Bob!
    B->>A: Hello Alice!

Loading

2. Generate SVG with transparent background:

npx mmdc -i diagram.mmd -o diagram.svg -b transparent

3. Inspect the generated SVG:

grep 'class="messageLine' diagram.svg

Expected Behavior

Message lines should use the CSS class definition:

<line class="messageLine0" stroke-width="2" marker-end="url(#arrowhead)" .../>

Where the CSS defines:

.messageLine0 {
    stroke-width: 1.5;
    stroke-dasharray: none;
    stroke: #333;
}

Actual Behavior

Message lines are rendered with inline stroke="none":

<line class="messageLine0" stroke-width="2" stroke="none" marker-end="url(#arrowhead)" .../>

The inline stroke="none" overrides the CSS class definition, making the lines invisible.

Impact

• Sequence diagrams appear broken with only actors visible
• Message lines and arrows are completely invisible
• Affects all v11.x versions tested (11.4.0, 11.10.1, 11.12.0)

Workaround

Post-process the SVG to remove the inline stroke="none" attribute:

const content = fs.readFileSync('diagram.svg', 'utf8');
const fixed = content.replace(
    /(<line[^>]*class="messageLine\d+"[^>]*)\s+stroke="none"/g,
    '$1'
);
fs.writeFileSync('diagram.svg', fixed, 'utf8');

Additional Context

• Flow diagrams render correctly without this issue
• The CSS class definition is present and correct in the generated SVG
• Only the inline attribute needs to be removed for proper rendering

Regression

This appears to affect all v11.x versions, suggesting it may have been introduced in the v11 rewrite.